The invention concerns a circuit arrangement for the compensation of the temperature coefficient of semiconductor junctions.
It is known that the relationship between the collector current Ic and the base emitter potential U.sub.BE of a transistor is strongly temperature dependent and it is further known that this temperature coefficient is much larger with small collector currents than with higher collector currents.
This is of particular concern in the exposure measuring and control circuits of photographic devices, wherein the measurement or the control of the shutter, respectively, is effected by means of the flow of a very slight current produced by the incident light in a photoelectric cell. The effect has recently been gaining additional importance, because the photoelectric cells are becoming steadily more sensitive, i.e. they work with constantly declining illumination levels, yielding correspondingly lower signal currents.
The operating range of high performance photographic cameras at the present time frequently extends to an exposure value of Ev-1 at 21 DIN or even lower. This corresponds to a luminous density of 0.06 cd/m.sup.2. Photographs of objects at night with a full moon are thus possible. Generally, cameras capable of measuring at such low luminances are equipped with Si photocell because CdS photo conductive cells, due to their sluggishness at low luminances, cannot be used in such cameras.
Si photo diodes produce at such low luminances signal currents of a few pA only. The measuring range is limited by the ratio of the signal current to the residual current.
When this ratio drops below an approximate factor of 10, accurate measurement of luminances is no longer possible. Because slight residual currents in particular are very strongly dependent on temperature, the range of measurements may be specified, strictly speaking, at a given temperature only.
It is, however, practically impossible for the user of such a high performance camera to decide, on the basis of brightness and temperature, whether an exposure is possible or not.
The use of highly-sensitive films and wide-opening lenses increases the danger that the user will make exposures below the minimum luminance, leading to incorrectly-exposed images. Thus, for example, with a luminance of 0.06 cd/m.sup.2, a 36 DIN film, and a lens with an aperture of 1:1.4, the exposure would be 1/15 sec., i.e., an exposure which under certain conditions may be effected freely by hand. In other fields of applications of these high-performance cameras, for example, operations photography, and microscope photography, the minimum luminance is also reached very quickly and the user should in such case be warned.
There exists, however, the difficulty that the currents provided by the photo diode, because of their wide dynamic range, must be compressed. This is done, for example, with the aid of a transistor in a logarithmic circuit. Thereby arises the problem that the nearly temperature-independent photo cell current is converted into a base-emitter potential of a transistor, which is strongly dependent on temperature.
Such a circuit could be extended to include a warning device operable at the smallest permissible current level, by incorporating a comparator having one input supplied with the base-emitter potential U.sub.BE of the logarithmic transistor and another input provided with an adjusted potential from a voltage divider. This circuit has, however, the inherent disadvantage that its indication is correct only at the temperature for which it is has been balanced.
To obtain correct indications from such a warning device, it would be necessary to take the reference voltage not from a temperature-independent resistive divider, but from a divider having the same temperature dependence as the base-emitter of the logarithmic transistor at the smallest photocell current to be indicated. This may be effected either by means of an expensive network of thermistors and balancing trimmers for the reference voltage, requiring a relatively long time for balancing, or by taking the reference voltage from a semiconductor element of the type used in the logarithmic stage. The difficulty here lies in that the semiconductor element must be supplied with a current as small as the smallest photocell current to which the warning device is to respond. This is likewise very difficult to obtain. In addition, the comparator to be used would require a very high impedance input, for example, a FET input.